This invention relates to a control valve for hydraulically actuated load positioning devices and particularly to a flow control valve of a counterbalance construction.
Hydraulically driven carrier apparatus is used in various industrial applications for transport and positioning a load. The speed of the carrier apparatus may be accurately controlled by proper control of the flow rate of the hydraulic fluid through a reversible hydraulic motor means. The system advantageously may use a suitable hydraulic pump to supply operative hydraulic fluid under pressure to the motor. The output of the pump is selectively connected to the load positioning apparatus through a directional control valve to control the direction and the speed of movement. Under positive load conditions, a full flow connection may advantageously be supplied to the load positioning device. However, under negative load conditions, the flow control system must prevent undesired and uncontrolled rapid movement of the load. Typically hydraulic motors are of a linear piston-cylinder construction. Rotary hydraulic motors are also employed and particularly as traction power source for mobile equipment, such as off-the-road vehicles.
The hydraulic motors, of course if over driven such as by gravity forces, function as a pump and will in fact partially evacuate the fluid input side of the motor. Such action can result in complete loss of control, particularly loss of braking action with a resulting runaway condition. Further, the operator cannot possibly anticipate the changing load conditions to accurately control the flow so as to prevent undesirable motion. For example, in earth moving equipment, such as bucket loaders, crane devices and the like, the load member is biased by PG,3 gravity for dropping or lowering movement. In such applications as well as others, the load under certain conditions may overrun the supply and become a pump device thereby creating a negative load condition. In the absence of compensating means, the load may drop in a more or less uncontrolled manner. Such uncontrolled or unrestricted movement of the load may not only severly damage the equipment but may harm surrounding personnel. Various control valves have been suggested in which the negative load condition in a hydraulically system is monitored and controlled. For example, the recently issued U.S. Pat. No. 4,018,136 which issued Apr. 17, 1977 to Lorel D. Kaetterhenry discloses a control valve for hydraulically driven apparatus, such as earth moving and construction equipment and the like. The control valve shown therein is a dual section valve including a pilot operated check valve having a pressure sensitive pilot operator coupled to the drop power end of the power cylinder. The valve unit is actuated to permit a relatively full flow to and from the power cylinder to raise the bucket or move the load. During lowering, if a negative load condition is generated, the spool valve responds to close and develop a restricted flow. The valve includes a second pilot operated safety valve which is connected in series with the spool valve having a similar pressure sensitive pilot operator. In the event of a precipitous change in the pressure conditions, the safety valve is operated to close and in essence to hydraulically lock the fluid motor in position to prevent precipitous dropping movement of the load. The system thus provides a pair of series connected pilot operated control valve in combination with a pilot operated safety valve which continuously relies on the pressure differential of the hydraulic motor.
Fixed orifice flow regulators control the flow rate to minimize the runaway condition of a hydraulic motor under negative loads. However, a fixed orifice regulator tends to provide a damp oscillation with a chattering type of movement. Piston-type systems in many instances do not provide the necessary response and change with pressure condition in a time delay response with a corresponding chattering or erratic movement of the load. Improved pilot operated valve structure have been suggested in which the response is more accurately and fully controlled in response to flow conditions including piloted response to both motor directional drives or a combined unit having a check valve flow for lifting combined with pilot operated lowering.
For example, recently issued U.S. Pat. No. 4,051,864 which issued Oct. 4, 1977 discloses a flow regulating valve for again controlling the supply of hydraulic liquid to a similar hydraulic power cylinder, shown applied to the lifting mechanism of a fork lift unit. As noted therein various problems have been noted in the control of construction equipment and other similar lifting devices such as fork lift trucks to create safe, reliable and smooth operation.
Further, the control valves should provide essentially instantaneous and smooth response to the changing pressure conditions as well as compensating for such changes in order to maintain a smooth controlled movement of the load. Further if the supply is shut off to both sides of the motor, hydraulic positioning and holding of the load in place is established. This of course requires reliable trapping of the hydraulic liquid to the opposite sides of the motor.
Generally, it has been found extremely difficult to provide a positive, reliable control while simultaneously maintaining the system lock condition.
When hydraulic holding of the load is provided, problems may be encountered, for example, in response to temperature variations, and even excessive load conditions. The pressure can build up in the system beyond the mechanical capability of the housing and passageway system. If such occurs a failure in the system may result in unexpected and uncontrolled release of the load.
Although various other patents have also been considered, the problem associated with the control of flow to and from hydraulic motors subject to negative loading and/or requiring a hydraulic holding require special consideration to produce the desired smooth movement of the load under both positive and negative loading conditions.